Most bed pillows are made of poultry down or feathers or a combination of both, and more recently, they have been made of synthetic plastic foam materials of various weights and degrees of firmness. In general, such pillows have a uniform interior density and thus provide a uniform degree of support for the user that is generally fixed at the time of manufacture.
However, for persons who have been injured or are in pain, particularly in the neck area, it is desirable to be able to adjust the firmness of the pillow in different areas so as to maximize comfort.